1. Field of the Invention
The present invention relates to a data receiving system such as mobile communications using equalizers and the like, and more particular, to a data receiving system which improves the equalizing capability in the radio communication channel subjected to multipath fading and hence improves the reception quality.
2. Prior Art
In mobile communications, buildings or similar constructions are obstacles reflecting the radio waves and separating the propagating route of the radio waves into a plurality of different transmission paths connecting the transmitter side and the receiver side. It is accordingly usual that an early-arriving signal coming along a shorter transmission path and a late-arriving signal coming along a longer transmission path are mixed and inter-symbol interference is caused therebetween. In view of the above-described problem, digital mobile communication data receiving systems comprise adaptive automatic equalizers to compensate such inter-symbol interferences and enhance their reception quality. Among numerous equalizers, a decision feedback equalizers (DFE) is widely used because of its small computation amount. The decision feedback equalizer is an adaptive automatic equalizer which calculates an equalized output by using reception data entered after a designated time and the decision value of reception data entered before the designated time.
In several digital mobile communication systems such as PDC (personal digital cellular) and GSM (global system of mobile communications), a sync signal is positioned at the center of a burst and interposed between transmission data (refer to 21, 22 and 23 of FIG. 8). The data receiving system, receiving the transmission data of such a format, comprises two decision feedback equalizers independently performing the adaptive equalization processing of decision feedback type. One of two decision feedback equalizers performs the equalization for the data succeeding the sync signal, while the other decision feedback equalizer performs the equalization for the data preceding the sync signal.
This kind of conventional data receiving system, as shown in FIG. 7, comprises a receiving buffer 1 storing reception data, a forward training calculator 3 which calculates an initial value of the tap coefficient used in the equalization for the reception data succeeding the sync signal based on the training using the sync signal (known word) involved in the reception data, a backward training calculator 4 which calculates an initial value of the tap coefficient used in the equalization for the reception data preceding the sync signal based on the training using the sync signal, a forward tracking calculator 7 which performs the DFE equalization processing for the data succeeding the sync signal by using the tap coefficient (forward tap coefficient) 5 sent from forward training calculator 3, and a backward tracking calculator 8 which performs the DFE equalization processing for the data preceding the sync signal by using the tap coefficient (backward tap coefficient) 6 sent from backward training calculator 4.
According to this data receiving system, reception data II are stored in receiving buffer 1 and are occasionally used for the computation in each of calculators 3, 4, 7 and 8.
Forward training calculator 3, as shown in FIG. 8, performs the training of estimating the channel condition based on the difference between the known sync word and the actually received data, in the entire region of a predetermined forward training section 24 which is equivalent to the reception section of the sync signal. The forward tap coefficient 5, obtained through this training, is sent to forward tracking calculator 7.
Forward tracking calculator 7 performs the adaptive equalization processing of decision feedback type for the data II (23) received after sync signal 22, in the entire region of forward tracking section 25, by using an initial value equal to the forward tap coefficient 5. Then, forward tracking calculator 7 generates a demodulation output 9.
Meanwhile, backward training calculator 4 performs the training in the entire region of a predetermined backward training section 26 by retroactively following up the sync signal 22 along the direction opposed to the time sequence in the reception. The backward tap coefficient 6, obtained through this training, is sent to backward tracking calculator 8.
Backward tracking calculator 8 performs the adaptive equalization processing of decision feedback type for the data I (21) received before sync signal 22 along the direction opposed to the time sequence in the reception, in the entire region of a predetermined backward tracking section 27, by using an initial value equal to the backward tap coefficient 6. Then, backward tracking calculator 8 generates a demodulation output 10. The forward demodulation output 9 sent from forward tracking calculator 7 and the backward demodulation output 10 sent from backward tracking calculator 8 are taken out as a demodulation output 11 in a time division manner.
FIG. 9 shows the impulse response of the radio communication channel. Case (a) represents a so-called minimum phase transition where the power of a leading wave is larger than that of a delayed wave. Case (b) represents a so-called non-minimum phase transition where the power of the leading wave is smaller than that of the delayed wave. The DFE-type equalizer can compensate the distortion derived from the delayed wave in each of the minimum phase transition and the non-minimum phase transition. Accordingly, the above-described data receiving system can perform the equalization processing for both of data I (21) and data II (23), improving the reception quality.
However, the way of removing the inter-symbol interference in the above-described decision feedback equalizer is to cancel the delayed signal when signals are successively entered. For this reason, in the case of the minimum phase transition, the one not canceled and remaining is the signal having a larger power and the removed one is the signal having a smaller power. On the contrary, in the case of the non-minimum phase transition, the remaining one is the signal having the smaller power and the removed one is the signal having the larger power. The S/N ratio is hence deteriorated. Accordingly, the minimum phase transition is preferable to obtain a larger effect of the DFE, when it is compared with the non-minimum phase transition.
However, the above-described conventional receiving system performs the equalization processing for both of data I and data II of the reception data by inverting the time sequence in a reciprocative manner. For this reason, either the data I or data II must be the minimum phase transition and the other must be the non-minimum phase transition whenever the basic wave and the delayed wave are both present. Accordingly, the fact that the half of the reception data is always the non-minimum phase transition provokes the problem that the degree of improvement of the reception quality is substantially determined by the effect to the non-minimum phase transition.